Technical Field
The present invention relates to a chuck mechanism for detachably holding a rotary tool at a head part of a dental handpiece, specifically a micro-motor hand-held piece and further to a dental handpiece using the chuck mechanism.
Background Art
A dental micro-motor hand-held piece is designed such that a columnar rotary tool for cutting teeth (called contra-bar or latch bar) is detachably attached on a head part and axially rotates by rotary drive force of a micro-motor (rotary drive portion) embedded in the hand-held piece body. Several kinds of rotary tools are prepared and a suitable one is selected from them by an operator depending on the state of affected region and is used by attaching it on the head part. When the tip part (cutting operation part) of the rotary tool becomes worn, the tool is exchanged to a new one. Therefore, the rotary tool is designed so as to be easily attached or detached by a chuck mechanism incorporated into the head part. Such a chuck mechanism for a dental handpiece and a chuck device are disclosed in Patent Literatures 1 and 2, for example.
Patent Literature 1 discloses a chuck device for holding a contra-bar by locking a lock member to a locking groove formed at the upper end side (opposite to a cutting operation part, namely a base end part) of the contra-bar, the lock member being provided on the upper part (at the base end part side) of a bar tube (rotor) which receives and fixes the contra-bar and being capable of elastically reaming on a surface area orthogonal to the axial center of the bar tube. In the prior art (FIG. 11, FIG. 12) shown in Patent Literature 1, the chuck device is positioned on an inner diameter side of an upper bearing rotatably holding the bar tube. In the embodiment, it is pointed out as a problem that elastic function of the lock member cannot be adequately exerted even when the lock member is designed to be able to elastically ream (enlarge) in a limited space. Therefore, Patent Literature 1 proposes a chuck device as a new invention in which a lock member having similar function (of which structure is different) is positioned on the upper part of the upper bearing where a relatively larger movable space can be obtained for elastically reaming the lock member on a surface area orthogonal to the axial center of the bar tube.
Patent Literature 2 discloses a dental handpiece in which a manual button constituting a chuck release means is operated by pressing against the elastic force of a spring, the lower surface of the manual button abuts a ball provided on an upper surface of a pusher, and a chuck mechanism is released via the pusher by such abutting pressure.
PTL 1: JP-H04-90752-A
PTL 2: JP-2006-346452-A
The chuck device disclosed in PTL 1 as a new invention adequately exerts elastic function of the lock member; however, the head part becomes bulky in the axial center direction of the bar tube because the chuck device is positioned on the upper part of the upper bearing, so that such a device may not be suitable for a dental handpiece which is used to be inserted into narrow oral cavity.
FIG. 14a, FIG. 14b, and FIG. 14c diagrammatically show one embodiment of a chuck mechanism in practical use. FIG. 14a is a sectional view of the chuck mechanism shown with a rotary tool held with the chuck mechanism. FIG. 14b is a perspective view of a chucking member constituting the chuck mechanism. FIG. 14c is a left side view of FIG. 14a. The chuck mechanism 100 has a chucking member 101 shown in FIG. 14b. The chucking member 101 comprises a cylindrical base part 101a externally fitted to a shank part 102a of a columnar rotary tool 102 and a pair of elastic chucking pieces 101b in the shape of tongue having a chucking pawl 101c which axially extends from the cylindrical base part 101a and is elastically locked to a locking groove 102c formed at a base end part 102b of the rotary tool 102. A D-cut part 102d is further formed at the base end part 102b of the rotary tool 102 (refer to FIG. 14c). The tip part extending from the shank part 102a of the rotary tool 102 (opposite to the base end part 102b) is a cutting operation part 102e which projects from a head part, not shown, and is used for cutting teeth. A rotor 103 is externally fitted to the shank part 102a of the rotary tool 102. The rotor 103 has a tool guide tube part 103a externally fitted to the shank part 102a, a connection tube part 103b which is connected to the tool guide tube part 103a and is externally fitted to and integrated with the cylindrical base part 101a of the chucking member 101, and a driven-transmitted gear portion 103c which is formed on the outer circumference of the tool guide tube 103a. 
In addition, a tubular rotor outer member 104 is externally fitted to and integrated with the connection tube part 103b of the rotor 103 and has a receiving part 104a formed corresponding to the shape of the base end part 102b having the D-cut part 102d of the rotary tool 102. When the base end part 102b is received in the receiving part 104a and they are engaged, mutual rotation of the rotor outer member 104 and the rotary tool 102 is made impossible. Namely, the rotor for axially rotating the rotary tool 102 is substantially constituted by the rotor 103 and the rotor outer member 104. Bearings 105, 106 are fitted and attached on the outer circumference of the tool guide tube part 103a of the rotor 102 and on the outer circumference of the base end part of the rotor outer member 104, respectively. The rotor outer member 104, the rotor 103, the chucking member 101, and the rotary tool 102 are integrally held to the head part, not shown, in an axially rotatable manner via the bearings 105, 106. A tapered cam-follower surface 101d which reams in the centrifugal direction is formed on the centrifugal side of the chucking pawl 101c of the chucking member 101. When a push button, not shown, attached on the head part, is operated along the axial direction to the cam-follower surface 101d, the elastic chucking piece 101b is pushed and enlarged against the elastic force and the chucking pawl 101c is made disengageable from the locking groove 102c. When the chucking pawl 101c is disengaged from the locking groove 102c, the rotary tool 102 can be removed from the rotor 103.
In the chuck mechanism 100 of the rotary tool 102 constituted as in FIG. 14, the elastic chucking piece 101b of the chucking member 101 extends in the axial direction from the cylindrical base part 101a, so that the elastic deformation is not limited and locking and disengaging functions of the chucking pawl 101c to and from the locking groove 102c can be adequately exerted. However, the cylindrical base part 101a of the chucking member 101 is integrated in the inner circumference of the rotor 102, so that there is a problem that circumscribed length L1 along the axial direction of the tool guide tube part 103a to the shank part 102a of the rotary tool 102 is not adequately secured. The inner diameter of the tool guide tube part 103a is made slightly larger than the outer diameter of the shank part 102a considering the insertion ability of the rotary tool 102. When the circumscribed length L1 is not largely secured, the rotary tool 102 and the rotor 103 cause axial runout, although the rotary force of the rotor 103 is directly transmitted to the rotary tool 102 in the circumscribed length L1. If the hand-held piece is used for a long time when they cause axial runout, cut powder caused by the cutting operation part 102e is accumulated and solidified between the shank 102a and the tool guide tube part 103a, as a result the rotary tool 102 is not able to be removed.